Described below is a method and a telecommunication device for selecting a number of code channels and an associated spreading factor for a CDMA transmission between a sending and a receiving device in the field of mobile telecommunications. More specifically, the determination of parameters for a CDMA (code division multiple access) transmission is targeted.
Wireless transmission relies on a transmission medium with resource limitations. The available bandwidth has to be allocated in an efficient way to allow for many-user transmission services. More conventional ways for resource allocation are based on the division by frequency (FDMA: frequency division multiple access) or division by time (TDMA: time division multiple access). GSM (Global System for Mobile communications) avails of these technologies. A novel approach is provided by CDMA (Code Division Multiple Access). Instead of dividing the allowed frequency range into a few hundred narrow channels, CDMA usually allows each station to transmit over the entire frequency spectrum all the time. Multiple simultaneous transmissions are separated using coding theory. In CDMA, each bit time is subdivided into short intervals called chips. Each station is assigned a unique code called chip sequence. To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the one's complement of its chip sequence. All chip sequences are pairwise orthogonal, so that the signal of a given sender can be extracted via the inner product of the received signal with the sending station's chip sequence. Thus, each station can use the full spectrum.
CDMA requires selecting transmission parameters (spreading factor) and used physical resources (number of time multiplexed parallel code channels) in a CDMA telecommunication device. After rate matching the coded information bits are mapped to the available physical resources in such way that PAPR (Peek-to-Average Power Ratio) is kept low by limiting the number of parallel code channels (avoiding of multi-code). In addition, the degradation by the influence of inter symbol interference is kept low by avoiding the usage of the lowest spreading factor unless a critical puncturing ratio is exceeded for higher spreading factors. Thereby, the spreading factor usually id defined as the ration between chip rate and data rate, or, equivalently, as the ratio of the information bit duration over the chip duration.
In communication systems physical resources and transmission parameters are often determined depending on the number of information bits after coding and the available physical resources which can depend on the capability of the telecommunication device. One example of such a transmission system is the so called E-DCH (Enhanced Dedicated Channel) scheme which is an enhancement of UMTS (Universal Mobile Telecommunication System).
The outlines of the E-DCH standard can be found in the 3GPP RAN Specifications and the scheme for selecting the physical layer resources is described in the Specification TS 25.212 V6.6.0 in Chapter 4.8.4.1 “Determination of SF and number of PhCHs needed”. This specification explicitly sets out how to determine or select a number of code channels and an associated spreading factor for a CDMA transmission from those supported by the hardware. Selection criteria are the necessary amount of puncturing and the number of code channels. Puncturing, a procedure used to combine the power-control bits and the binary voice symbols on traffic channels, means to discard voice symbols in favour of inserted power-control bits. Obviously, too much puncturing will lead to a degradation of the voice signal. As a rule, puncturing increases with the spreading factor. In determining a number of code channels and an associated spreading factor one principally proceed as follows. There are limits to the maximum amount of puncturing. These limits will define an allowable puncturing range corresponding to numbers of code channels and associated spreading factors. This range may also be limited in view of necessary repetitions of information bits. As a further criterion one uses the number of code channels, which should be as low as possible. Last, among those pairs of code channel and associated spreading factor left (which will only differ with respect to the spreading factor) one chooses the pair with the smallest spreading factor so as to decrease the amount of puncturing as much as possible.